The present invention relates to a protective relay circuit for selectively operating on interphase faults. More particularly, the present invention is directed to a protective relaying circuit which utilizes symmetrical component values to permit tripping of three poles of a breaker selectively on interphase faults only, and selectively discriminates against three pole tripping on single line to ground faults.
In the past, it has been known that under certain conditions it is advantageous to be able to trip a single pole of a three phase alternating current power transmission line, and under other circumstances to be able to trip all three poles of the three phase line. Where a single line to ground fault occurs internal to the line being protected, it is highly desirable to be able to trip only a single pole. Where interphase faults occur, such as double line to ground faults, phase to phase faults and three phase faults, it is highly desirable to be able to accurately and rapidly detect such conditions and to generate a three pole trip signal which enables the tripping of the three poles of a circuit breaker, assuming that other signals supplied to the three phase trip control circuit indicate that the fault is internal to the line being protected. In the past, appropriate pole trip selection for various types of faults was based on signals from a combination of phase and ground distance relays. However, this arrangement was not always satisfactory since the ground distance relays would not always detect fault conditions where there was a high resistance fault. Another method of approaching the problem of the detection of interphase faults on a three phase power transmission line has been directed to the detection of differences between the magnitudes of the voltages of the three phases on the line. However, this method suffers from the disadvantage that the differences in the voltages may not be substantial where there is a relatively small source impedance, even though an interphase fault has occurred on the line.
The present invention utilizes the fact that certain predetermined per unit symmetrical component voltages occur at a fault depending upon the nature of the fault. The theory of symmetrical components is well understood by those skilled in the art of electric power transmission and distribution. In accordance with this theory, the phase currents or voltages in any unbalanced three phase alternating current electric circuit can be resolved into three sets of symmetrical, balanced current or voltage vectors known respectively as the positive sequence component, the negative sequence component and the zero sequence component. The negative and zero sequence components of circuit currents and voltages are not present under balanced circuit conditions. A more detailed discussion of the theory of symmetrical components may be found in the textbook, Symmetrical Components, by Wagner and Evans, published by McGraw-Hill in 1933.